1. Field of the Invention
The present invention relates to heterogeneous nanowires having a core-shell structure consisting of single-crystal apatite as the core and graphitic layers as the shell and to a synthesis method thereof. More specifically, the present invention relates to a method capable of producing large amounts of heterogeneous nanowires, composed of graphitic shells and apatite cores, in a reproducible manner, by preparing a substrate including an element corresponding to X of X5(YO4)3Z which is a chemical formula for apatite, adding to the substrate a gaseous source containing an element corresponding to Y of the chemical formula, adding thereto a gaseous carbon source, and allowing these reactants to react under optimized synthesis conditions using chemical vapor deposition (CVD), and to a method capable of freely controlling the structure and size of the heterogeneous nanowires and also to heterogeneous nanowires synthesized thereby. The examples of the present invention show the results of using calcium or strontium as an element corresponding to X of the chemical formula, the results of using phosphine containing phosphorus for the formation of Y of the chemical formula, and the results of using acetylene (C2H2), methane (CH4), ethylene (C2H4) or propane (C3H8) as a gaseous carbon source for the formation of graphitic layers.
2. Description of the Prior Art
Crystalline carbon materials such as graphene are known as advanced materials having very excellent electrical, mechanical, chemical and physical properties and have recently been used in a wide range of applications, including electrode materials, reinforced composite materials, and optical materials.
Carbon nanowires are advanced materials having a shell structure consisting of graphene rolled up into a cylinder shape having a diameter of nanometer order and can be broadly classified into carbon nanotubes, carbon nanofibers, and carbon nanocables. Carbon nanocables generally refer to structures in which materials having the shape of rods or wires are included in the hollow spaces of graphitic shells, unlike carbon nanotubes or carbon nanowires.
When the material included in such carbon nanocable structures to form the core is disadvantageous that it is sensitive to external environmental factors (for example, it is easily to be oxidized, or is adversely affected by acid or is sensitive to water), or the mechanical and physical properties are inherently weak or the electrical properties thereof are not excellent, the graphitic shells surrounding the surface of the core materials can overcome such disadvantages, and thus are highly beneficial.
In addition to the fact that the inherent properties of the core material can be maintained by the graphitic shells, the properties of the core material which is very excellent in one of the electrical, mechanical, chemical and physical properties can be additionally improved by the graphitic shells.
Meanwhile, carbon nanocable structures can be formed by various methods. The most general method is an in-situ formation method that is based on chemical vapor deposition (CVD) or arc discharge. In this method, cores which can be present in carbon nanotubes are made mainly of transition metals having an excellent catalytic activity of forming carbon nanotubes. In recent years, nanosized metal oxides have been reported to be able to form such graphitic shells. The vapor-liquid-solid (VLS) growth mechanism is primarily responsible for the synthesis of carbon nanocables by the above method.
Another method is a method of filling a liquid or gaseous material into prepared carbon nanotube structures using capillary action, a wet-chemical method, and a nano-filling reaction. In this method, mass production is not easier than in the in-situ formation method, but there is an advantage in that various materials can be used as the core material.
Meanwhile, it has not yet been reported that bio-minerals such as apatite can be formed directly into carbon nanotube structures which comprise, for example, graphitic shells. Calcium phosphate compounds are typical minerals and can typically be developed to the chemical structure of apatite, which is generally represented by X8(YO4)3(Z), wherein X may represent Ca, K, Na, Sr, Ba, Mg, Pb, Cb or Zn, Y may represent P, As, V or S, and Z may represent OH−, F−, CO3− or Cl−. Compounds represented by the chemical formula have various properties and structures depending the components and composition ratios of X, Y and Z.
Particularly, compounds in which X is Ca and Y is P are calcium phosphate compounds which have various properties and structures depending on whether Z represents OH−, F−, O−, CO3− or Cl−. Specific examples of the calcium phosphate compounds include hydroxyapatite: HA(Ca/p=1.67)-Ca5(PO4)3(OH); fluoroapatite: (Ca/p=1.67)-Ca5(PO4)3(F); carbonated apatite: (Ca/p=1.67)-Can(PO4)8(CO3)(OH), oxyapatite: OA(Ca/p=1.67)-Ca10(PO4)6O; octacalcium phosphate: OCP(Ca/p=1.33)-Ca8H2(PO4)65(H2O); tricalcium phosphate: OCP(Ca/p=1.5)-Ca3(PO4)2; tetracalcium phosphate: OCP(Ca/p=2.0)-Ca4(PO4)2O; brushite: (Ca/p=1.0)-CaH(PO4)2(H2O); and monetite: (Ca/p=1.0)-CaH(PO4)).
These compounds generally have very excellent biocompatibility and are used mainly in the biotechnology field related to the production of artificial teeth and bones, but are known to have low mechanical strength and insufficient electrical and chemical properties.
In addition, the synthesis of calcium phosphate compounds is generally carried out in a moisture- or oxygen-rich atmosphere because of their structural characteristics. Such conditions for the synthesis of calcium phosphate compounds are significantly inconsistent with conditions for the production of graphitic structure, and thus two kinds of materials (graphitic shells and calcium phosphate compounds) were difficult to synthesize simultaneously under the same conditions. Accordingly, there is a need to form composites of calcium phosphate compounds and graphitic nanostructures, thereby improving the mechanical and physical properties of the calcium phosphate compounds.